1. Field of the Invention
This invention relates to multi-color electrochromic flat-panel displays embodying solid-state electrochromatic cells with solid polymer electrolytes. The electrochromic material, which is soluble in water associated with or a water emulsion of an ionically conducting polymer electrolyte, changes color in response to small applied anodic or cathodic currents providing writing and erasure processes as a result of anodically or cathodically applying voltages to the cell. On open-circuit, the cell retains its state of charge or memory. The multi-color electrochromic cells using solid polymer electrolytes according to the present invention provide a display which is thin, compact, lightweight, sunlight readable, possessing open circuit memory, operable over a wide operating temperature range requiring low power input for operation.
2. Description of the Prior Art
Electrochromic effects of various chemicals and various chemical systems wherein specific chemicals undergo chemical reactions in response to small applied anodic or cathodic currents changing their color are known. Color changes, rates, and performance characteristics, are greatly dependent upon the electrochromic chemical used and the entire electrochromic system including the electrolyte and cell configuration. Electrochromic reactions of rare earth diphthalocyanines have been reported by P. N. Moskalev and I. S. Kirin, Opt. Spectrosc., 29, 220 (1970) and P. N. Moskalev and I. S. Kirin, Russ. J. Phys. Chem., 47, 1019 (1972). Color changes in a lutetium diphthalocyanine film on tin oxide in an aqueous electrolyte of KCl or Na.sub.2 SO.sub.4 is described in M. M. Nicholson and F. A. Pizzarello, Charge Transport in Oxidation Product of Lutetium Diphthalocyanine, J. Electrochem Soc., 127, 2490 (1979) and M. M. Nicholson and F. A. Pizzarello, Galvanostatic Transients in Lutetium Diphthalocyanine Films. The cathodic and anodic electrochromism of lutetium diphthalocyanine films have been found dependent upon the specific aqueous and organic liquid electrolyte as well as other specific cell components as reported by M. M. Nicholson and F. A. Pizzarello, Cathodic Electrochromism of Lutetium Dipthalocyanine Films, J. Electrochem Soc., 128, 1740 (1981). Electrochromic action of other rare earth diphthalocyanines are recognized to be similar and electrochromic changes of lutetium and ytterbium diphthalocyanines are very similar in a number of aqueous and organic electrolyte liquids. D. Walton, B. Ely and G. Elliott, Investigations into the Electrochromism of Lutetium and Ytterbium Diphthalocyanines, J. Electrochem Soc., 128, 2479 (1981). The use of lutetium diphthalocyanine with aqueous and organic liquid electrolytes in electrochromic displays is described in M. M. Nicholson and T. P. Weismuller, Multicolor Electrochromic Display Technology, Technical Report No. 5, Office of Naval Research Contract N00014-77-C-0636, Task No. NR 359-667 (1983). Different electrochromic behavior of lutetium diphthalocyanine films in single boundary and dual boundary cell configurations using aqueous liquid electrolytes and emphasizing the need for aqueous electrolytes to promote ion transport is described in M. M. Nicholson and T. P. Wiesmuller, Evidence of Electronic Conduction Due to Mixed Oxidation States in Lutetium Diphthalocyanine Films, Report No. 7, Office of Naval Research Contract N00014-77-C-0636, Task No. NR 359-667 (1985).
The electrochromic action of 4,4'-dipyridinium compounds known as viologens in aqueous and organic liquid electrolyte systems is known. The action of n-heptylviologen in various salt liquid solutions showed dependence upon specific anions, cations and metal presence. R. J. Jasinski, The Electrochemistry of Some n-Heptylviologen Salt Solutions, J. Electrochem Soc., 124, 637 (1977). Large differences in redox potentials between ethylviologen and benzylviologen in aqueous and organic liquid solvents have been noted. H. T. van Dam and J. J. Poujee, Electrochemically Generated Colored Films of Insoluble Viologen Radical Compounds, J. Electrochem Soc., 121, 1555 (1974). The electrochromic changes of diheptylviologen film on tin oxide electrodes in an aqueous electrolyte are described in J. Bruinink and C. G. A. Kregting, The Voltammetric Behavior of Some Viologens at SnO.sub.2 Electrodes, J. Electrochem Soc., 125, 1397 (1978). Photoreduction of aqueous solutions of heptylviologen bromide on p-GaAs in the form of photoelectrochemical cells is described in B. Reichman, F. F. Fan and A. J. Bard, Semiconductor Electrodes, J. Electrochem Soc., 127, 333 (1980). The differing conductance of helptylviologen in aqueous and organic solvents is described in H. T. van Dam, The Conductance of Heptylviologen Dibromide in Water and Methanol, J. Electrochem Soc., 123, 1181 (1976).
Solid-state electrochromic displays are taught by U.S. Pat. No. 4,550,982 wherein an organic electrochromic material and an ionic material are provided in a polymeric layer. The organic electrochromic material is associated with the polymeric material either as integrated macromolecules or polymer molecules, or as pendant macromolecules by covalent bonding, or is dispersed in the polymeric layer.
Other electrochromic displays using solid polymer electrolytes are described in Japanese Patent Application Publication No. 84/37804, Matsushita Electric Industrial Co. Ltd., using pyridinium materials and in Japanese Patent Application No. 82/223451, Publication No. 84/113422, teaching a solid acrylic resin tetrathiofulvalene having a lithium borofluoride polymer display.